Integral waterwall external heat exchangers

ABSTRACT

A combustion system having a combustor that includes tubing for carrying boiler water, a cyclone to recover solids from exhaust of the combustor and an external heat exchanger to recover heat from the solids, includes: a bypass for providing boiler water from the combustor to tube bundles of the external heat exchanger and a boiler water return for providing boiler water from the tube bundles to waterwall tubing of the combustor. A method and an external heat exchanger are also provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention disclosed herein relates to boiler evaporative surfaceand, in particular, to an integral boiler waterwall external heatexchanger in a fluid bed boiler.

2. Description of the Related Art

Consider a combustion system that makes use of an external heatexchanger.

Referring to FIG. 1, there are shown aspects of an embodiment of a priorart circulating fluidized bed steam generator (CFBSG) 10. In thisembodiment, the CFBSG 10 includes a combustor 2, at least one cyclone 3,a respective seal pot 4, a respective external heat exchanger (EHE) 5,and various other components. For purposes of illustration, it isconsidered that the exemplary embodiment makes use of only one cyclone3. However, it applies to any number of cyclones.

In operation, crushed fuel (e.g., coal) and sorbent (e.g., limestone)are fed to a lower portion of the combustor 2. Primary air is suppliedto a bottom portion of the combustor 2 through an air distributor, withsecondary air fed through one or more elevations of air ports in a lowerportion of the combustor 2.

Combustion takes place throughout the combustor 2, which containscirculating bed material. Flue gas and entrained solids leave thecombustor 2 as combustor exhaust and enter one or more cyclone units(simply referred to as the “cyclone” 3). In the cyclone 3, solids areseparated from the flue gas and fall to a seal pot 4. From the seal pot4, the solids are recycled to the combustor 2 via an ash return 19. Inthis embodiment, some of the solids are diverted to the external heatexchanger (EHE) 5 and then to the combustor 2 via an EHE outlet duct 23.In the EHE 5, one or more tube bundles absorb heat from the fluidizedsolids to cool the fluidized solids and to convert part of the waterflowing through the tube bundles into steam, which is supplied to auser, such as a turbine for power generation. Solids travel between theEHE 5 and the seal pot 4 by an EHE inlet duct 22.

In some lighter duty circulating fluidized bed steam generator (CFBSG)10, the EHE 5 is not warranted. However, in certain other embodiments,such as those taking advantage of certain reheat cycles, fuels, or of acertain steam capacity, use of the EHE 5 is advantageous. Typically, theEHE 5 is deployed as a bubbling-bed heat exchanger that includes one ormore compartments, each compartment including an array of immersed tubeswhich are grouped as at least one tube bundle 6.

In typical embodiments, the combustor 2 generally includes two regions.A lower portion and an upper portion. The lower portion of the combustor2 includes the fuel, a primary air distributor, secondary air ports,fuel feed ports and solid recycle ports. The density of the bed in thisregion is relatively high on average and typically highest at theelevation of the air distributor. The density then drops off withincreasing height of the combustor 2. Physically, the lower portion isusually rectangular, tapered and formed from finned or fusion weldedwaterwall tubing 15. The lower portion is typically lined withrefractory to protect the waterwall tubing 15.

In this simplified illustration, the waterwall tubing 15 is suppliedboiler water (water from a drum) by an inlet header 17. The inlet header17 provides the boiler water for steam generation in the waterwalltubing 15 of the combustor 2.

The upper portion of the combustor 2 includes at least one gas outlet 7which communicates with the cyclone 3. The upper portion is usuallyrectangular with vertical walls, where the walls are formed with finnedor fusion welded waterwall tubing 15. The upper portion is typicallyunlined to maximize heat transfer to the waterwall tubing 15.

The walls of the combustor 2 are cooled by thermo-syphonic (natural)circulation. At high steam and water pressures, the walls of thecombustor 2 may incorporate assisted circulation.

In the arrangement design of evaporative tubing in a boiler, the heatabsorbing surface must be configured as to avoid dry out of the internalsurface of the tubing. This is accomplished by preventing steam andwater separation. The separation is prevented by having all heatedcircuits run vertically or sloping upward with rifled tubing, or bybeing pumped horizontally. Otherwise the tubing will either becomeoverheated due to a lack of coolant, or else it will quickly suffer highinternal disposition rates of iron oxide, with the potential foroverheating and internal corrosion. Therefore, in evaporative bundleEHE's, either inclined/vertical heat absorbing surface is used, or elsepumped horizontal surface.

Still referring to FIG. 1, the prior art CFBSG 10 includes a boiler drum10 which receives saturated steam and water from the waterwall tubing 15through various outlet header and riser tubes. The boiler drum 8provides for separation of water (W) and steam (S). In this embodiment,the drum water (W) is also directed to an EHE circulating pump 16 via adowncomer 9. Steam is provided from the EHE 5 via an evaporative tubeoutlet header (ETOH) 93 to an evaporative bundle riser 95. The steamfrom the EHE 5 is directed into the steam coming from the combustor 2.

The design presented in FIG. 1, and other similar designs, requires theuse of a circulating pump 16, which use electrical energy, in order toprovide adequate water flow to the circuits to avoid internal dry out.Also, the EHE circulating pump 16 is an expensive piece of equipmentthat can require regular maintenance. Further, other components arerequired, such as the EHE downcomer 9, EHE Evaporator risers, EHEevaporator Inlet and Outlet headers.

To avoid the use of an EHE circulating pump 16, an EHE tube bundle 6 canuse natural circulation by inclining the tubes and using rifled tubing.In order to install adequate surface in an external evaporative heatexchanger (such as the EHE 5) while still maintaining naturalcirculation, a deep bundle is required in order to provide significantupward slope needed to promote the water circulation in each circuit.Upward slope is required to avoid dry out in each tube. The deep bundlerequires a deep bed which uses a higher pressure blower (in the case ofa external heat exchanger) with a higher power requirement per unit airflow than would be required for a shallower bed, as would be the casewith horizontal tubing and pumped (assisted) circulation. The inclinedsurface also results in less surface area for the tube bundle 6 per unitbed plan area with the consequent requirement of more evaporatorassemblies and greater plan area.

Therefore, what is needed are design features for an EHE evaporator thatprovide for reduced space and bed height or elimination of severalcomponents, and elimination of circulation pumps.

BRIEF SUMMARY OF THE INVENTION

Disclosed is a combustion system having a combustor including tubing tocarry boiler water, a cyclone to recover solids from exhaust of thecombustor and a heat exchanger to recover heat from the solids, thesystem including: a bypass for providing boiler water from the combustorto tube bundles of the heat exchanger and a boiler water return forproviding boiler water from the tube bundles to waterwall tubing of thecombustor.

Also disclosed is a method for providing boiler water to a heatexchanger in a combustion system that has a combustor including tubingfor carrying boiler water, a cyclone to recover solids from exhaust ofthe combustor and a heat exchanger to recover heat from the solids,where the method includes: directing boiler water from the combustorinto a bypass and providing the boiler water to tube bundles of the heatexchanger.

In addition, a heat exchanger is provided and includes an input adaptedfor receiving a bypass tube that provides boiler water from waterwalltubing of a combustor and tube bundles for heating the boiler water andretruning the boiler water to the combustor via return tubes incontinuous circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 depicts aspects of a prior art circulating fluidized bed steamgenerator;

FIG. 2 depicts aspects of the circulating fluidized bed steam generatorsystem (CFBSGS) according to the teachings herein;

FIG. 3 depicts a side view of an external heat exchanger (EHE);

FIG. 4 depicts a rear view of the EHE;

FIG. 5 depicts a view of a waterwall from inside of a combustor;

FIG. 6 depicts a plan view of the EHE in relation to the combustor;

FIG. 7 depicts tube offset between the EHE and the combustor;

FIG. 8 depicts aspects of the CFBSGS; and

FIG. 9A, and FIG. 9B, collectively referred to as FIG. 9, depictadditional embodiments for providing boiler water to the combustor andtube bundles of the EHE.

DETAILED DESCRIPTION OF THE INVENTION

Disclosed is an improvement to a circulating fluidized bed steamgenerator (CFBSG) 10. The teachings herein provide for, a simplerexternal heat exchanger, with less power requirements than a typicalexternal heat exchanger, and a reduced number of components in thecirculating fluidized bed steam generator system CFBSGS 100.

Referring now to FIG. 2, showing aspects of an example of the invention.This depiction is simplified for purposes of illustration. Not all ofthe components typically associated with a fluidized bed combustor areshown.

In the illustration, a portion of the boiler water for the waterwalltubing 15 is diverted to the EHE 5 and back to the waterwall. As shownin FIG. 2, a fraction of the waterwall tubes are diverted to form tubebundles 6 of the EHE 5. The EHE 5 functions in much the same way as inthe prior art (with some differences and advantages including thosediscussed herein as well as others) to provide for recovery of thermalenergy from the combustor solids. Thermal energy recovered by use of theEHE 5 is carried by return flow via return tubes 21 from the EHE 5 tothe waterwall tubing 15. The return tubes 21 provide a continuous flowpath for the return flow from the tube bundles 6 and back to theremaining waterwall tubing 15. The essential point of the invention isthat the vertical flow heat absorption in the combustor ensures adequateflow to avoid dry out or overheating for the part of each tube that ishorizontal in the EHE.

As used herein, waterwall tubing 15 is defined as the water cooledtubing forming the containment perimeter of a combustion chamber. Thewaterwall tubing 15 provides for collection of heat produced in thecombustor 2 and generation of the steam.

In other embodiments, water is diverted from the combustor inlet headerto separate inlet header for the tube bundle. The selected tubes for thecombustor are left out. The tubes of the bundle crossover to thewaterwall filling in the missing tubes.

Consequently, at least a portion of the boiler water for the waterwalltubing 15 is used to absorb heat by using the EHE 5. The extent of theportion may be determined according to thermal performance requirements;and may include some, or up to all, of the boiler water. That is, insome embodiments, all of the boiler water is directed through the bypass20. In addition, one skilled in the art will recognize that the bypass20 may be used in conjunction with prior art designs, such as thoseusing an EHE circulating pump 16 and various downcomers 9 to providewater from the steam drum 8. A variety of advantages are realized by useof the bypass 20 (and the associated components).

As in the prior art, at least one downcomer 9 supplies water to theevaporative cycle from the steam drum 8. As used herein, the term“downcomer” is defined as a pipe carrying boiler water from the steamdrum 8 to the inlet of the evaporative surfaces. Flow direction in thedowncomer 9 is downward, and there may be distribution devices such asheaders, pipes and tubes between the downcomer and the water walls.

Though a circulating pump 16 may be used to circulate water in theevaporative cycle, natural circulation is preferred for simplicity andlower cost. Under “natural circulation” water circulates through theevaporative cycle (e.g., the various water components described hereinthat are fed by the drum and return the water steam mixture to the drum)unaided by a separate pump that requires power.

As used herein, “natural circulation,” also referred to as “thermalcirculation” or “thermosyphonic circulation” is the process ofcirculating boiler water through the closed loop consisting of the steamdrum 8, downcomers, waterwall and risers of boiler components. Thedriving force for the circulation is the difference in hydrostatic headbetween the downcomer 9 and the heated waterwalls.

Several advantages are realized by integrating the circuit of the EHEwith that of the waterwall. One advantage is insuring naturalcirculation in the EHE even with horizontal heat absorption surface.This reduces the required height of the fluidized bed, which, in turn,reduces the fluidizing air pressure requirements in comparison withsloped evaporation surface. Accordingly, use of primary air, secondaryair, and or a source of other lower pressure air, to maintain fluidizedbed conditions is possible. The fluidizing air can enter the combustor 2as secondary air. Thus there is no greater fan power requirement thanwithout an external heat exchanger.

Furthermore, the EHE circuits do not have separate risers or downcomers,since they are part of the waterwall circuits. Thus, the total amount ofpressure parts is reduced in comparison to either a typical externalheat exchanger or an evaporative panel.

Consider further aspects of the prior art. With an in-combustor surface,such as wingwalls, the extent of evaporative surfaces is fixed. There islittle control over temperature in the combustor 2 when process changesoccur (such as those that are due to variations in fuel or limestonequality and sizing). Exemplary complications include changes in heattransfer rate, sulphur capture and other such concerns. In short, theamount of fixed surface may be too little or too great to achieve theoptimum process temperature for a given condition. In contrast, flowcontrol of solids return from the sealpot 4 through the EHE inlet duct22 to the EHE 5 by use of control valve permits users to increase ordecrease evaporation during operation, and to thus achieve a largemeasure of control over the combustor bed temperature during operation.

Controlling combustor evaporation and operating temperature may beperformed in addition to other techniques for controlling the process,such as reducing bed inventory or changing ratios of primary air tosecondary air. The greater complexity of performance and emissions withvarying fuel and limestone qualities require ever increasing numbers ofindependent process control variables, which cannot be addressed by aCFBSGS 100 having a fixed surface.

In the basic embodiment discussed above, a fraction of the waterwalltubes 15 from a section of the combustor 2 are run outside the combustor2 into a separate EHE, (which could be fluidized or not) where each tubeforms a separate element of a heat exchanger tube bundle 6. The enclosedarea of the EHE can be either water cooled or refractory lined.Circulating CFB ash is directed to the EHE from the sealpot 4 (alsocalled a siphon seal and a J-leg). An opening in the enclosed areaallows the ash to be returned to the combustor 2 either directly, orindirectly through another fluidized area, such as a sealpot 4 or airslide. Each heat absorbing tube 6 of the EHE is run back to thecombustor 2 and is used as a waterwall tube 15.

In an embodiment also discussed above, the tubes do not originate in thewaterwall, but in an external header that is supplied with water from adowncomer 9. Each tube forms a separate element of a heat exchanger tubebundle 6. As in the previous example, each heat absorbing tube of theEHE is run back to the combustor 2 and used as a waterwall tube 15.

Exemplary diagrams showing further aspects of the CFBSGS 100 are nowprovided. In FIG. 3, the enclosure of EHE 5 is a refractory lining 34.In other embodiments, the enclosure is formed from waterwall tubes 15 ofthe combustor 2. Also shown in FIG. 3 are expansion joints 33, the tubebundle 6, an EHE ash inlet duct 22, EHE fluidizing air headers and pipes32, as well as evaporative bundle inlet tubes 24.

In FIG. 4, a rear view of the EHE 5 is provided. In the rear view, aportion of the inlet header 17 is shown. A number of the evaporativebundle inlet tubes 24 and tubes of the tube bundle 6 are shown. Theevaporative bundle inlet tubes 24 are fed by an EHE inlet header 92.

In FIG. 5, a portion of the waterwall which the integrated waterwallexternal heat exchanger 5 is joined is shown. In this illustration, thepenetrations include the ash return 19 and an air vent 52. The air vent52 is shown in an elevated position.

FIG. 6 provides a plan view of the combustor 2 and the EHE 5 in relationto each other. In this illustration, the combustor 2 is rectangular inform. FIG. 7 depicts an embodiment of the bypass 20 in relation to thecombustor 2 and the EHE 5. In this example, the tubes between thecombustor and external heat exchanger are shown with expansion loops. Inanother embodiment, the EHE is supported off the combustor and moves upand down with it.

FIG. 8 provides another illustration of relationships presented in FIG.2. In FIG. 8, a portion of the combustor 2 and a portion of the cyclone3 are shown. It should be noted that the cyclone 3 and the combustor 2bear a certain physical relationship to each other such that placementand use of the EHE 5 is most efficient. One skilled in the art willrecognize that numerous design parameters play a role in a final designfor the CFBSGS 100. Accordingly, it should be understood that theteachings herein are merely illustrative and are not limiting of theinvention.

FIG. 9 depicts further embodiments involving the bypass 20. In theseembodiments, at least one downcomer 9 provides boiler water from thesteam drum 8. Depicted in FIG. 9 is an EHE inlet header 92 (shown as asupply header in FIG. 2). The EHE inlet header 92 is illustrated forsimplicity and to explain the designs provided.

In the example of FIG. 9A, flow from the EHE header 92 and the downcomer9 passes into the evaporative inlet tubes 24 and then into the tubes ofthe at least one tube bundle 6. This arrangement provides for separatedowncomers 9. In this embodiment, one downcomer 9 (or set thereof) isdirected to the EHE 5, and the other downcomer 9 (or set thereof) isdirected to the combustor 2. In this design, the EHE 5 and the combustor2 are in parallel.

In another embodiment provided in FIG. 9B, a crossover link 91 isincluded. The crossover link 91 permits diverting a portion of the flowfrom the combustor inlet header 17 (show this on FIG. 9B) to theevaporative inlet tubes 24 via the EHE inlet header 92. In thisarrangement, the boiler water flows from the downcomer 9 to the EHEinlet header 92 and then to the combustor 2.

A third embodiment is shown in FIG. 9C. In this embodiment, thedowncomer 9 is in fluid communication with the combustor inlet header 17and evaporative supply tubes 31. The evaporative supply tubes 31 takeflow to the EHE inlet header 92. In this arrangement, flow from thedowncomer 9 goes to the combustor first, then to the EHE 5.

Accordingly, with reference to FIG. 9, one skilled in the art willunderstand that a variety of embodiments may be realized concerningdistribution of boiler water flow between the combustor 2 and the EHE 5as well as mixing flow from various collection points.

In some embodiments, since only a fraction of the waterwall tubes isused for the heat exchanger, the remaining wall tubes can still supportthe weight of the waterwall if the combustor is bottom supported or theweight of the plenum floor and bed, if the combustor is top supported.

Unique features provided for by the teachings herein include use of anexternal heat exchanger with horizontal, or substantially horizontal,evaporative surfaces (i.e., tubes) are possible for controllingtemperature in the combustor 2, using circulating ash to performevaporative heat duty without the need for sloping the tubing upward orincluding forced circulation.

Further advantages of the teachings herein over prior art bubblingexternal heat exchangers used for controlling bed temperature are thatno forced circulation or sloped natural circulation surface is requiredin order to assure circulation and avoid steam blanketing andoverheating of the tubes. The power and equipment expenses of forcedcirculation are avoided, and large bed volumes required with slopednatural circulation are avoided. Further, the need for separate relieftubes (risers or down corners) for the evaporative bundle are avoided.

As used herein, the terms “water,” “feedwater,” and “boiler water” makereference to liquid or coolant used for the thermodynamic cycle of theCFBSGS 100. It is recognized that the liquid or coolant is typicallywater, but that other constituents may be included. For example, theliquid or coolant may include chemicals for limiting erosion andcorrosion of various components. It is considered that all of these andother such liquids or coolants fall within the meaning of theseforegoing terms.

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications will be appreciated by those skilled in theart to adapt a particular instrument, situation or material to theteachings of the invention without departing from the essential scopethereof. Therefore, it is intended that the invention not be limited tothe particular embodiment disclosed as the best mode contemplated forcarrying out this invention, but that the invention will include allembodiments falling within the scope of the appended claims.

1. A combustion system including a combustor comprising tubing to carryboiler water, a cyclone to recover solids from exhaust of the combustorand an external heat exchanger to recover heat from the solids, thesystem comprising: a bypass for providing boiler water from thecombustor to a tube bundle of the external heat exchanger and a boilerwater return for providing boiler water from the tube bundle towaterwall tubing of the combustor.
 2. The system as in claim 1, whereinthe bypass comprises at least one of waterwall tubing and an inletheader.
 3. The system as in claim 1, wherein the bypass comprises aninput for receiving boiler water from a downcomer.
 4. The system as inclaim 1, wherein the external heat exchanger is adapted for naturalcirculation.
 5. The system as in claim 1, further comprising ash flowcontrols for controlling heat absorption of an evaporative surface thatis external to a combustor.
 6. The system as in claim 1, wherein thetube bundles of the external heat exchanger are at least one ofhorizontal and substantially horizontal.
 7. The system as in claim 1,wherein each tube in the tube bundle is coupled to, and in fluidcommunication with, a waterwall tube of a circulating fluidized bedsteam generator to provide the boiler water return.
 8. A method forproviding boiler water to a heat exchanger in a combustion systemincluding a combustor comprising tubing for carrying boiler water, acyclone to recover solids from exhaust of the combustor and an externalheat exchanger to recover heat from the solids, the method comprising:directing boiler water from the combustor into a bypass; and providingthe boiler water to tube bundles of the external heat exchanger.
 9. Themethod of claim 8, further comprising receiving the boiler water fromthe tube bundles and directing the boiler water to waterwall tubing ofthe combustor.
 10. The method as in claim 8, wherein providing theboiler water comprises absorbing heat in the boiler water.
 11. Themethod as in claim 8, wherein the directing comprises controlling ashflow over an effective evaporative surface of at least one of theexternal heat exchanger and the combustor.
 12. The method as in claim11, wherein controlling comprises directing combustor solids.
 13. Themethod as in claim 8, further comprising controlling an air supply tothe combustor.
 14. The method as in claim 8, wherein the directingcomprises directing the boiler water from a lower portion of thecombustor.
 15. The method as in claim 8, further comprising circulatingsolids from the cyclone in the heat exchanger.
 16. An external heatexchanger comprising: a tube bundle wherein each tube in the tube bundleis coupled to, and in fluid communication with, a waterwall tube of acirculating fluidized bed steam generator.
 17. The external heatexchanger as in claim 16, wherein at least a portion of the tube bundlesare at least one of horizontal and substantially horizontal.